1. Field of the Invention
This invention relates to methods of preparing molded thermoplastic articles, and more particularly, but not by way of limitation, to methods of preparing molded thermoplastic articles wherein radio frequency energy is utilized.
2. Description of the Prior Art
Methods of preparing molded articles from thermoplastic materials using radio frequency energy to heat the materials are well known. For example, U.S. Pat. No. 4,288,399 issued Sept. 8, 1981 discloses a method of preparing molded articles wherein an amine polarizing agent is utilized to sensitize a thermoplastic composition to microwave energy followed by the application of such microwave energy to the composition for a time sufficient to produce a moldable consistency therein. The polarizing or radio frequency energy sensitizing agent, when mixed with a thermoplastic composition, functions to render the resulting mixture susceptible, or at least more susceptible, to the heating effects of radio frequency energy.
Heretofore, it has generally been necessary to ball mill or otherwise intimately mix the radio frequency energy sensitizing agent used with the thermoplastic material for a prolonged period of time prior to subjecting the resulting mixture to radio frequency energy. Also, it has generally been necessary to utilize separate sensitizing, plasticizing and lubricating agents in thermoplastic compositions. Plasticizing agents improve the low temperature flexibility and other properties of the molded product, and lubricating agents improve extrusion and molding properties.
Foamed thermoplastic products have heretofore been produced by mixing a chemical blowing agent with the thermoplastic composition at a temperature below the decomposition temperature of the blowing agent, and subsequently heating the mixture by conductance, convection or mechanical work to a temperature high enough for the blowing agent to decompose and release gases. Because the thermoplastic composition with blowing agent is often heated prior to being placed in a mold, the process involves the risk that the blowing agent will prematurely decompose. When reinforcing fibers, filaments or mats formed from fibers or filaments (all referred to herein as fibrous materials) are incorporated in the thermoplastic material to improve the strength and other physical properties of the resulting foamed product, the often used initial heating of the thermoplastic composition to facilitate combining the blowing agent and fibrous materials therewith risks premature foaming and often results in non-uniform cellular structure or the collapse of the cellular structure due to wicking action by the fibrous materials.
By the present invention improved methods of preparing molded thermoplastic articles, both solid and foamed and with and without fibrous materials, are provided which obviate the above problems.